The present invention relates generally to a coating method and a coating apparatus. More particularly, the present invention relates to a method and an apparatus for simultaneously and uniformly applying plural coating liquids onto a flexible support using an extrusion type coating head to form a laminated structure.
Generally, a conventional method for coating a moving strip-like flexible support (hereinafter referred to simply as a web) with a coating liquid such as a magnetic coating liquid or the like has been hitherto practiced by employing, e.g., an extrusion type coating apparatus, a curtain flow type coating apparatus, a doctor blade type coating apparatus, or a slide coat type coating apparatus. Among the aforementioned conventional coating methods, a coating method employing an extrusion type coating apparatus makes it possible to uniformly coat a web with a coating liquid while forming a thin layer on the web. For this reason, the foregoing coating method has been put in practical use in various applications (See, for example, Japanese Patent Publication No. 1-46186, Japanese Patent Laid-Open Publication No. 60-238179 and Japanese Patent Laid-Open Publication No. 2-265672).
However, the extrusion type coating apparatus has problems in that good coating results can practically be attained only within narrow ranges of coating conditions, and, moreover, desired coating results cannot be achieved when the coating liquid has high viscosity, the coating liquid is coated on a web in such a manner as to form a thin film thereon, or coating is performed at a high speed.
Moreover, as the transport speed of the web increases, there arises a problem in that the quantity of air entrapped in the space between the web and a coating head rapidly increases, causing the film thickness to become irregular due to the entrapment of air bubbles in the coated film.
To obviate the foregoing problems, the coating head can be configured such that the liquid pressure at the outlet of the slit of the coating head is increased to prevent air from being entrapped between the coating head and the web, or the surface of a doctor edge can be smoothed to prevent stripe-like irregularities from appearing on the coated web due to the presence of foreign particles entrapped on the top of the doctor edge.
In another approach, an extrusion type coating apparatus is employed which includes a doctor edge surface a part of which projects from a tangential line at the downstream end of a back edge surface so as to prevent a part of the web from being cut at the downstream end of the back edge surface. Such an arrangement is disclosed in Japanese Patent Laid-Open Publication Nos. 1-184072, 1-210072, 1-288364 and 2-251265.
Recently, as magnetic recording media have increasingly been formed with multiple layers and recorded at high density, it has become necessary to coat a magnetic layer on a web in a small thickness. In the circumstances mentioned above, it has been more strongly required that the coating liquid be coated on the web at a higher speed for the purpose of improving productivity. In addition, as properties of the magnetic material have been improved, oxide-based magnetic powder and barium ferrite based magnetic material, each exhibiting a high S.sub.BET value, have been increasingly used. On the other hand, as a result of development efforts for recording a magnetic recording medium at a high density, it has become possible to coat a web with a magnetic coating liquid having high viscosity.
However, when a web is coated with a coating liquid at a coating rate of 10 cc/m.sup.2 or less employing an extrusion type coating unit wherein coating with coating liquids is simultaneously achieved at a high speed to form a laminated structure composed of an upper layer and a lower layer, there arises a problem in that stripe-shaped irregularities frequently appear in the coated layer. Another problem is that it sometimes becomes impossible to form the upper layer by coating.
To determine the causes of the aforementioned problems, the inventors have conducted a variety of experiments, as a result of which the following facts were found.
First, it is assumed that an imaginary straight line extends tangentially along the doctor edge surface of the coating head and another edge surface located on the upstream side of the former (back edge surface or doctor edge surface located upstream of the former) and a perpendicular line extends upward of the upstream end of the doctor edge surface. It is further assumed that the distance of the upstream end of the doctor edge surface from an intersection where the perpendicular line intersects the straight line is designated by t. When the distance t is small, foreign particles in the coating liquid are entrapped at the upstream end of the doctor edge surface, resulting in stripe-shaped irregularities frequently appearing on the coated layer. On the contrary, when the distance t is large, as coating liquids are simultaneously coated on the web to form a laminated structure, a boundary surface between both the upper and lower layers is adversely disturbed, whereby the upper layer cannot practically be coated on the web at a coating rate of 10 cc/m.sup.2 or less.